Cobalt(II) and iron(II) complexes containing various multi dentate Schiff base ligands with N4, N2O2, N2S2 and N2P2 donor sets are being evaluated as synthetic oxygen carriers. Thermodynamic, EPR and inert atmosphere electrochemical studies and X-ray techniques are being employed to study the electronic and/or structural factors which stabilize the dioxygen adducts of the species under evaluation.